1. Field of the Invention
The present invention relates to a system of supplying electric power to an induction furnace-provided with an ac generating apparatus that constitutes an exclusive-use power source, independent of a commercial power source, such as an ac power source for an induction furnace (or an induction heater), to which required heating electric power is supplied via its single-phase coil and which serves as a single-phase load with respect to its ac power source. An output voltage and frequency of the ac generating apparatus is continuously variably controlled.
2. Description of Conventional Art
FIGS. 4-6 show conventional induction-furnace power-supplying systems of this type. In such conventional systems, required heating power is supplied to the induction furnace.
For example, the system shown in FIG. 4 uses a commercial power source as a basic power source, and uses, an ac generator driven by a prime mover, such as a diesel engine, as a complementary power source for the commercial power source. The system shown in FIG. 5 uses the commercial power source as an exclusive-use power source. Because the system causes a motor generating apparatus driven by the commercial power source to function as a frequency converter, it serves as a required heating power source and is generally used for a high-frequency furnace. In addition, the system shown in FIG. 6 uses an ac generator driven by a prime mover, such as the one described below, as the exclusive-use power source. The system, change in the required supply voltage is effected by a changeover of a tap of a transformer mounted in a power-supplying main circuit. The frequency of the supply voltage is usually made identical to the commercial frequency. Alternatively an output of the ac generator is rectified and is then converted to an alternating current having a required voltage and frequency by an inverter to supply power.
In the following description of FIGS. 4 to 6, component elements having the same functions in the drawings are denoted by the same reference numerals or characters.
First, in FIGS. 4A and 4B, G.sub.3 denotes a three-phase ac generator. E denotes a prime mover such as a diesel engine for driving the generator. CB.sub.S1 and CB.sub.S2 denote power source-side circuit breakers. CB.sub.L1 to CB.sub.Ln (n=1, 2, . . . ), denote load-side circuit breakers. Numeral 10 denotes induction furnace facilities constituted by an induction furnace and its incidental equipment. COS in FIG. 4B denotes a changeover switch for separating the power-supplying main circuit between the commercial power source and the three-phase ac generator.
Namely, FIG. 4A shows a basic circuit configuration in which the Circuit is arranged to enable a generating apparatus, constituted by the prime mover E and the three-phase ac generator G.sub.3, to operate in parallel with the commercial power source, and which is used for peak cutting when maximum receiving power from the commercial source is restricted. Meanwhile, FIG. 4B shows a basic circuit configuration in which the generating apparatus is operated as an emergency power source for supplying power to the induction furnace facilities separated from the commercial power source by the changeover switch COS during a power failure of the commercial power source. In either FIG. 4A or FIG. 4B the generating apparatus is used as a complementary power source for the commercial power source.
Accordingly, with regard to the generating apparatus, its output frequency is identical to that of the commercial power supply, and in FIG. 4A its output capacity is set to be less than the difference between the required maximum power for the overall loads, including the induction furnace facilities 10, and the maximum contract power. Meanwhile, in FIG. 4B, the output capacity of the generating apparatus is determined, as required, by setting as its minimum value the sum of various power required for continuing the operation of the induction furnace in a heat-retained condition. In either case, the output capacity of the generating apparatus is set to be a value smaller than the aggregate total of the rated power of the aforementioned loads.
Next, in FIG. 5, M denotes an ac motor. G.sub.1 denotes a high-frequency single-phase ac generator driven by the motor. T.sub.R1 denotes a transformer. Numeral 7 denotes a single-phase coil for applying heating power mounted on the body of the induction furnace; C.sub.P denotes a power-factor improving capacitor for the single-phase coil. and Numeral 11 denotes induction furnace facilities in which the aforementioned single-phase coil and the aforementioned various power-supplying incidental elements are grouped together.
Namely, FIG. 5 shows an induction furnace power-supplying system that uses the commercial power source as its exclusive-use power source, and which is generally used for a high-frequency induction furnace. The motor M and the generator G.sub.1 together constitute a motor-generator that functions as a frequency converter with respect to a power-supply input from the commercial power source. It should be noted that the output voltage and the output frequency of the motor generator are rendered variable by adjustment of the energization of the generator G.sub.1 and adjustment of the number of revolutions of the motor M, respectively. In addition, the output capacity of the motor generator is determined as a value capable of supplying the required maximum power of the induction furnace.
In FIG. 6A, SW.sub.1 and SW.sub.2 denote switches of electromagnetic contactors or the like, respectively. CLR denotes a current-limiting resistor; T.sub.R2 denotes a tapped transformer. C.sub.B and L.sub.B denote a capacitor and a reactor, respectively, for phase balancing. 12 denotes induction furnace facilities in which the aforementioned single-phase coil 7 and the aforementioned various power-supplying incidental elements are grouped together.
Namely, FIG. 6A shows a basic circuit configuration of a power-supplying system for a low-frequency induction furnace that uses a generating apparatus constituted by the prime mover E and the three-phase ac generator G.sub.3 as its exclusive-use power source, and whose frequency is generally set to the 50/60 Hz of the commercial frequency.
It should be noted that the capacitor C.sub.P for improving the power factor is simply connected in parallel with the single-phase coil 7 and is designed to set the combined power factor of the two elements to 1 or a value close thereto and to allow the synthetic characteristic to serve as a resistance element. The parallel connection between the single-phase coil 7 and the capacitor C.sub.P, which are thus arranged like a resistance element, together with the phase-balancing capacitor C.sub.B and reactor L.sub.B, constitutes a phase-balancing Grebor circuit for balancing the loads of the power sources-side phases when power is supplied from the three-phase power source to the single-phase resistance load. In addition, when the resistance portion and the power factor of the aforementioned single-phase coil itself have changed in correspondence with the state of load of the aforementioned induction furnace, to balance the load among the phases on three-phase power source side as described above, the respective values of the elements of C.sub.P, C.sub.B, and L.sub.B are changed and controlled in association with a predetermined relationship through control of the opening and closing of a switch which operates in response to a command of an unillustrated power-factor and phase-balancing controller.
In addition, required heating power for the induction furnace, which is inputted via the single-phase coil 7, changes substantially in correspondence with the condition of operation of the induction furnace, such as heating, melting, and heat retention. The voltage to be applied to the aforementioned single-phase coil is changed and controlled by changing the taps of the transformer T.sub.R2 in accordance with a change of such required power, and the variable range of voltage reaches, for instance, approximately 20 to 100% of the rated voltage.
In addition, to control a transient overcurrent of the main circuit during the changing of the transformer taps, the insertion of the current-limiting resistor CLR into the main circuit by closing the switch SW.sub.2 with the switch SW.sub.1 open, the short-circuiting of that current-limiting resistor by closing the SW.sub.1 after completion of the state of the transient overcurrent of the main circuit current, and the setting of the current-limiting resistor in a parallel-off state by subsequently opening the SW.sub.2, are effected in a predetermined order.
Next, in FIG. 6B, T.sub.R3 denotes a transformer for a rectifier. REC denotes a rectifier circuit that is comprised of a plurality of rectifier elements respectively subjected to phase control, and which renders an output dc voltage thereof continuously variable. DCL denotes a dc rector for smoothing. INV denotes an inverter serving as a frequency converter. T.sub.R4 denotes a matching transformer. C.sub.P denotes a power-factor improving capacitor for the single-phase coil. Numeral 13 denotes induction furnace facilities in which the aforementioned single-phase coil 7 and the aforementioned various power-supplying incidental elements are grouped together.
Namely, FIG. 6B shows a power-supplying system which uses a generating apparatus constituted by the prime mover E and the three-phase ac generator G.sub.3 as its exclusive-use power source, and in which power supplied to the induction furnace is rendered continuously variable via a voltage transforming circuit and a frequency converting circuit whose outputs are respectively continuously variable. The power-supplying system of this type is generally used for high-frequency induction furnaces.
It should be noted that, in terms of its configuration, the power-supplying system shown in FIG. 6B is equivalent to a configuration in which the motor generating apparatus comprised of the motor M and the high-frequency single-phase ac generator G.sub.1 in FIG. 5 is substituted by a voltage/frequency converting circuit of a stationary type having a wider range of variable output. The supply voltage may be either three phase or single phase.
The variable range of required heating power of the induction furnace is generally required to be very extensive in the light of the diversity of its operating condition. Therefore, it is desirable that the voltage and frequency of electric power supplied to the induction furnace be controlled so as to be continuously and smoothly variable in an extensive range.
However, the various conventional induction-furnace power-supplying systems such as those described above have presented the several problems.
First, with respect to the power-supplying systems shown in FIGS. 4A and 4B, the subject induction furnaces are restricted to a low-frequency furnace to which the commercial frequency is applied. In addition, as a problem similar to that in the power-supplying system shown in FIG. 6A, if the configuration of the induction furnaces facilities 10 shown in FIGS. 4A and 4B are similar to the induction furnace facilities 12 in FIG. 6A, the change of heating power for the induction furnace is effected in stages by the changing of the taps of the transformer T.sub.R2, so that an amount of minimum change of the heating power naturally had to be restricted.
In addition, the induction furnace serves as a single-phase load with respect to its power source, and in a case where the power source is a three-phase ac power source, the provision of a phase balancing means becomes necessary to suppress the generation of a negative-phase-sequence component resulting from an interphase load unbalance due to the supply of power to the single-phase load. For this reason, the following become necessary: the power-factor improving capacitor C.sub.P of a large capacity for correcting the lagging power factor of the single-phase coil 7 of a low power factor; the capacitor C.sub.B and the reactor L.sub.B for phase balancing; a multiplicity of switches and a switch controller for the switches so as to render the aforementioned elements C.sub.P, C.sub.B, and L.sub.B continuously variable in accordance with a predetermined relationship, these elements being, in reality, arranged in step-like combinations of their unit amounts, respectively, in response to the condition of operation of the induction furnace. Hence, the configuration of the power-supplying system has been complex and large in size.
Furthermore, since the transformer T.sub.R2 opens and closes of the main circuit for transforming the supply voltage in the power-supplying system having the large-capacity capacitors such as C.sub.P and C.sub.B, the rush current into the main circuit during the closing of the main circuit in a state in which no measure is taken becomes excessively large, i.e., 15 to 18 times as large as the rated current thereof. Hence, to control that overcurrent, it is necessary to provide an overcurrent controlling means comprised of the switches SW.sub.1 and SW.sub.2, the current-limiting resistor CLR, and the like shown in FIG. 6A. At the same time, with respect to the capacity of the three-phase ac generator G.sub.3, in order to reduce a voltage drop due to the overcurrent after control and to absorb a negative-phase-sequence component due to the residual component of the interphase load unbalance, a value which is, for instance, 1.5 times the capacity corresponding to its required load capacity must be set as its rated capacity. Consequently, the ac generator becomes large in size, and the configuration of the power-supplying system is made more complex.
Next, with respect to the power-supplying system shown in FIG. 5, since the single-phase ac generator G.sub.1 is used for the induction furnace, which is a single-phase load, the configuration of the power-supplying system is quite simplified, but the size of that generator becomes very large as compared with the three-phase ac generator of the same capacity. Hence, a generator having a capacity capable of supplying the required maximum power for the induction furnace is bound to be very uneconomical. In addition, the power-supplying system basically uses the aforementioned commercial power source as an exclusive-use power source. Thus, its operation has been bound to be impossible during a power failure of the commercial power source.
Thus, although the power-supplying system shown in FIG. 6B is sophisticated, the system configuration is also complicated. In addition, to prevent the efflux of harmonics generated from each stationary-type converter to the power source side, it is necessary to dispose an unillustrated harmonic filter at an effective position such as at an input terminal of the induction furnace facilities 13. Also, with the three-phase ac generator G.sub.3, it is necessary to increase its capacity to such a degree that permits the absorption of the equivalent negative-phase-sequence component due to the aforementioned harmonics with respect to the required load capacity. Hence, it has been unavoidable that conventional systems are larger in size.
As described above, in the respective conventional systems for supplying electric power to an induction furnace, there have been no systems that optimally combine size, required installation space, price, and the like in terms of their functions and the configuration of the power-supplying system.